Correlations between free volume characteristics and pervaporation permeability of novel PVA–GPTMS hybrid membranes

Novel organic–inorganic hybrid membranes composed of poly(vinyl alcohol) (PVA) and γ-(glycidyloxypropyl)trimethoxysilane (GPTMS) were prepared through in situ sol–gel approach. The chemical structure and reaction mechanism for PVA–GPTMS hybrid membranes were tentatively proposed based on FTIR and solid-state 29Si NMR spectra. The free volume characteristics of PVA–GPTMS hybrid membranes were revealed by positron annihilation lifetime spectroscopy (PALS), and the apparent fractional free volume (fapp) were calculated according to free volume cavity size and intensity of o-Ps. The unusual pervaporation properties of PVA–GPTMS hybrid membranes for separating benzene/cyclohexane mixtures were discovered. The permeation flux increased from 20.3 g/(m2 h) for GPTMS-free membranes to 137.1 g/(m2 h) for PVA–GPTMS-04 membrane, while separation factor increased from 9.6 to 46.9 simultaneously. The correlation between fapp and permeation flux was established and the improvement in permeability reflected hybridization-induced disruption of polymer chain packing and an accompanying elevated free volume available for molecular diffusion.